[magick-users] Serious Magick++ slow-down in underlying Reduce()

[Elie Zedeck RANDRIAMIANDRIRAY]

> Color reduction is slowed considerably when an image has an alpha channel.
> Set image.matte(false) and the processing time should be decreased.

I didn't need to do that with my older method. So, I think, there is
really something not well going on.

I tested with IM 6.8.3-3 (provided by Gentoo portage) and it is now a
lot faster. It now renders about 5 - 8 seconds, while it rendered almost
30 seconds with the latest 6.4.1-1.

But still, that is slower than before.